Micro-CT

Micro-CT (or μCT) is, in essence, a scaled-down version of clinical CT. Micro- CT was developed in the early 1980s to overcome limited spatial resolution of clinical CT scanners when imaging small animals and biopsy-sized specimens from larger animals and humans.14 With the increasing interest in such imaging, micro-CT has

rapidly evolved into an efficient minimally-invasive method for high-resolution studies of micro-anatomy; this method provides relatively short scan times, isotropic volume

coverage, excellent sensitivity to skeletal tissue, and good sensitivity to soft tissue, especially when contrast-enhancing media are employed.15

A typical micro-CT scanner consists of a microfocus x-ray tube, a support for the object being imaged, and a high-resolution detector (Fig. 2.4). The microfocus x-ray tube has the focal-spot size in the range of 10-100 μm and usually incorporates a stationary tungsten anode.14-16 The high-resolution detector is either an x-ray image intensifier, as in

Microfocus x-ray tube Object Object support High- resolution detector Computer Controller

Figure 2.4. Typical micro-CT scanner.

Source: Adapted with permission from Elsevier. D. W. Holdsworth and M. M. Thornton, “Micro-CT in small animal and specimen imaging,” Trends Biotechnol. 20, S34-S39, 2002.

early micro-CT systems, or a scintillating screen optically connected (via a lens or a fiber-optic taper) to a charge-coupled device (CCD), as in the majority of current scanners.14,15 The use of a cooled CCD with fiber-optic coupling represents the most sensitive x-ray detection approach available today.17 Some recent designs of micro-CT scanners utilize flat-panel arrays as the high-resolution detector.17-19 The pixel spacing of the detector is generally around 50 μm or less.16 This spacing, along with the focal-spot size and system magnification, influences the scanner spatial resolution. The microfocus x-ray tube and the high-resolution detector can be either stationary, with the rotating object, or mounted on a rotating gantry, with the fixed object.20 Projection data acquired by the detector are fed into a computer, where they are used to reconstruct a CT image. The scanner also includes a controller (or controllers) to assure proper operation of the x- ray tube and the rotating device.

As implied by the above description, the typical micro-CT scanner employs the cone-beam approach for data collection. This approach is preferable for micro-CT due to maximum x-ray tube utilization and much faster volumetric acquisition.21 Although the single-slice geometry is still used in less time-sensitive applications, the cone-beam approach seems to become a de facto standard in the area. In addition to providing volumetric coverage, the cone-beam approach serves as a means of achieving the system magnification needed to exceed the inherent spatial resolution of the detector.14,20

Because of the resulting 3D data set, micro-CT utilizes a cone-beam reconstruction algorithm. The original method, known as Feldkamp (or FDK) algorithm, is an

approximation of 3D filtered backprojection.22 Despite recent introduction of many other methods, including exact solutions, the Feldkamp algorithm remains the most widely

employed cone-beam reconstruction technique due to its straightforward implementation and applicability to practical systems.15

Several variations of micro-CT scanners have been developed. The described design, which is based on a conventional x-ray tube source, is known as a bench-top micro-CT scanner.14 This design has been implemented in two configurations, for either

in vivo or in vitro imaging.16 The bench-top micro-CT scanners for in vivo imaging are used to scan small animals. To avoid soft tissue distortions and movements during the scan, the animal in these systems is kept in a fixed horizontal position, while the gantry rotates around the animal, much like in clinical units.14,20,23 Because the in vivo scanners are primarily optimized for minimum radiation dose to the live animal, they have

relatively low, as for micro-CT, spatial resolution, typically only 50-100 μm

isotropically.16 Their scan time, however, is rather short, usually less than 10 min. The FOV of such scanners ranges from about 50 to 100 mm. Due to modest spatial resolution, the in vivo systems are also referred to as mini-CT scanners.

The other configuration of bench-top micro-CT scanners, for in vitro imaging, is utilized to scan small specimens. In those systems, the specimen rotates around its vertical axis, but the x-ray tube and detector are kept stationary.14,20,24 The in vitro scanners are generally optimized for maximum spatial resolution, which is chosen to approach the resolution of histological microscopy. As a result, such scanners reach isotropic spatial resolution of around 10-50 μm.14-16 Their typical scan time is 10-30 min, and they commonly have the FOV of 15-50 mm. For many applications, the bench-top micro-CT scanners for in vitro imaging offer a cost-effective alternative to traditional microscopy and histology methods when studying microstructure of biopsy-sized

specimens.

Although some bench-top micro-CT scanners can achieve spatial resolution as high as 5 μm, even better resolution is provided by the synchrotron-based design.14 This design employs a monochromatic synchrotron x-ray source and a detector usually in the form of a scintillator-CCD couple. Because of the parallel x-ray beam geometry, the synchrotron-based micro-CT scanners cannot use the cone-beam approach to magnify the projection image. Instead, the magnification is accomplished by either optical coupling between the scintillator and the CCD, x-ray focusing via zone plates, or wavelength- specific x-ray diffraction in a Bragg magnifier.14,20,25 The synchrotron-based micro-CT scanners have spatial resolution of 1 μm or higher, allowing imaging of sub-cellular structures.25,26 Despite their superior resolution, the availability of such systems is limited, due to their dependence on synchrotron radiation sources.

As the size of an object being imaged and the voxel dimensions in micro-CT are much smaller compared with clinical scanners, several physics and technology aspects become important. First, because of the smaller object size, micro-CT requires lower x- ray photon energy, typically less than 25 keV.14,27 _{In this energy range, x-ray photons}

interact with matter primarily via the photoelectric effect, whereas in clinical scanners, in which the photon energy generally exceeds 50 keV, the main interaction mechanism is Compton scatter. A desirable feature of the photoelectric effect is considerably higher and more atomic-number-dependent x-ray attenuation, which permits much better tissue discrimination in micro-CT. The drawback, however, includes stronger dependence of that attenuation on the photon energy. Due to such dependence, micro-CT scanners are very sensitive to the x-ray photon energy, which must be adjusted according to the object

size to achieve a maximum signal-to-noise ratio (SNR).15,20,27 A further consequence of the strong energy dependence is higher susceptibility of micro-CT to beam-hardening artifacts; thus, nearly monochromatic radiation should be used to minimize these artifacts. Another important aspect of micro-CT scanners relates to their voxel dimensions being much smaller than in clinical systems. Smaller voxels require very efficient detectors, longer acquisition times, and higher radiation doses to achieve reasonable image quality.23,28-30 Also, because of the volumetric acquisition, smaller voxels in micro-CT lead to a huge amount of information (several gigabytes) in a typical data set.16,23,26 To store, process, and reconstruct this information, large computer

resources and longer execution times are needed. The final aspect of micro-CT scanners is a result of scaling-down the system design. Due to a smaller size, all scanner

components must be machined with higher accuracy, and all mechanical movements (especially in rotating-gantry scanners) must be performed with greater stability and precision.14,23

In summary, micro-CT is a rapidly developing field stimulated by the increasing demand for small-animal and small-specimen imaging. At its current stage, micro-CT offers isotropic spatial resolution from about 100 to 10 μm for bench-top scanners and even higher, up to sub-micrometer resolution for synchrotron-based systems. Micro-CT demonstrates high sensitivity to skeletal tissue and reasonably good sensitivity to soft tissue; the soft-tissue discrimination can be enhanced by a contrast agent. The scan times in micro-CT are comparable to those in other high-resolution imaging modalities.